This invention relates to a binder composition for binding foundry sand for forming molds and cores for use in a sand molds casting process and also to a resin coated sand prepared by using the same binder composition and a process of preparing the same resin coated sand.
In current sand mold casting processes, molds and cores are usually formed of a resin coated sand for which the use of a phenol resin is the most prevailing. Molds and cores formed of a phenol resin coated sand are high in physical strength after curing of the coated resin by a baking procedure which can be completed in a short time. However, the use of a phenol resin as the binder material for resin coated sand has offered some problems to sand mold casting processes. First, ammonia gas is liberated and accordingly an irritating odor is emitted during the baking procedure to complete the forming of the molds and cores because of partial decomposition of hexamethylenetetramine which is usually used as a cross-linking agent for a phenol resin. Second, in some alloy casting processes characterized by relatively low pouring temperatures, for example in aluminum alloy casting processes, the molds and cores, particularly the latter, retain their toughness even at the shake-out stage after solidification of the poured molten metal partly because the phenol resin undergoes partial carbonization by the heat of the molten metal with the result that the sand particles adhere strongly to each other or to the alloy casting, and therefore the cores cannot easily be disintegrated.
To solve such problems we have already proposed in our prior U.S. Patent Application Ser. No. 967,541 filed Dec. 7, 1978 now U.S. Pat. No. 4,246,165 to prepare a resin coated sand by using a crystalline unsaturated polyester resin as a binder material in place of a phenol resin. The moulds and cores formed of the proposed resin coated sand do not give out an irritating odour during baking and, as a more important feature, even cores used in aluminum alloy casting processes can easily be disintegrated at the shake-out stage.
However, unsaturated polyester resins are inherently lower in physical strength than phenol resins, so that in the case of using an unsaturated polyester resin often it becomes necessary to increase the proportion of the resin to foundry sand to be coated than in the case of using a phenol resin.